Field of the Invention
The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train of an automatic transmission for a vehicle that improves power delivery performance and fuel economy by achieving at least nine forward speed stages and widening speed ratio (or gear ratio) with a minimum number of constituent elements being used, and secures linearity of step ratios.
Description of Related Art
Recent increases in oil prices are triggering stiff competition among auto-makers in enhancing fuel consumption of a vehicle.
Such an automatic transmission achieving more speed stages is necessary to maximize power performance and driving efficiency according to downsizing of an engine. Particularly, it is desperately needed to develop high efficiency multiple-speed stage transmissions having excellent linearity of step ratios that can be used as an index closely related to drivability such as acceleration before and after shift, and rhythmical engine speed in order to secure competitiveness of automatic transmission field.
However, in the automatic transmission, as the number of speed stages increase, the number of internal components increase, and as a result, mountability, cost, weight, transmission efficiency, and the like may still deteriorate.
Accordingly, development of a planetary gear train which may achieve maximum efficiency with a small number of components is important in order to increase a fuel efficiency enhancement effect through the multiple-speeds.
In this aspect, in recent years, 8-speed automatic transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
However, since a conventional 8-speed automatic transmission has a gear ratio span of 6.5-7.5 (gear ratio span is an important factor for securing linearity of step ratios), improvement of power performance and fuel economy may not be great.
In addition, if an 8-speed automatic transmission has gear ratio span larger than 9.0, it is hard to secure linearity of step ratios. Therefore, driving efficiency of an engine and drivability of a vehicle may be deteriorated, and thus, development of high efficiency automatic transmissions which achieve at least nine forward speed stages is necessary.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.